Today, mobile information terminals such as mobile phones, laptop computers, and PDAs have been rapidly enhanced in functionality and compactness and reduced in weight. As the driving power sources for these terminals, nonaqueous electrolyte secondary cells represented by lithium ion secondary cells, which have high energy density and high capacity, are widely used. However, in recent years, with further enhancement in the functionality of these appliances, there has been a need for further enhancement in the capacity of the cells.
In view of this, as a measure to enhance the capacity of the cells, such a technique is proposed that cell capacity is increased by, for example, charging the positive electrode to a potential higher than 4.3V to enhance the use efficiency of the positive electrode active material. For example, such a technique is proposed that a positive electrode active material in which lithium cobalt oxide with different elements added therein and lithium nickel-manganese oxide having a layer structure are mixed with one another is used (e.g., patent document 1).    [Patent document 1] Japanese Patent Application Publication No. 2005-317499.
In these techniques, the structural stability of the lithium cobalt oxide during charging at a potential higher than a positive electrode potential of 4.3 V (based on lithium) is improved by adding different elements such as Zr and Mg to the lithium cobalt oxide. Also thermal stability on a high potential level is improved by providing the lithium nickel-manganese oxide with a layer structure. Such a technique is proposed that by using a mixture of these two compound oxides, stability during high voltage charging is enhanced. Also such a technique is proposed that by adding lithium phosphate to the positive electrode, a charge/discharge property at high voltage is improved.
However, these techniques pose the following problems. If these techniques are used, although the resistance of the positive electrode active material against high voltage charging is enhanced, the electrolyte is oxidatively decomposed at the positive electrode side during high voltage charging, thereby compromising cell preservation performance and a cycle property. Also, there is such a problem that especially under high temperature conditions, cobalt is eluted into the electrolyte solution and precipitates on the surface of the negative electrode, thereby degrading the preservation property and the cycle property.
Prior art techniques related to improvement in a charge/discharge cycle property are disclosed in, for example, patent documents 2 to 4.    [Patent document 2] Japanese Patent Application Publication No. 63-152886.    [Patent document 3] Japanese Patent Application Publication No. 2003-308842.    [Patent document 4] Japanese Patent Application Publication No. 2005-71641.
Patent document 2 discloses such a technique that in a nonaqueous electrolyte secondary cell comprising: a negative electrode having lithium or an alloy containing lithium as an active material; a positive electrode having manganese dioxide as an active material; and a nonaqueous electrolyte solution composed of a solvent and a solute, a mixture solvent containing 1,3-dioxane is used as a solvent for the nonaqueous electrolyte solution. According to this technique, although the charge/discharge cycle property can be improved because, for example, the growth of lithium dendrite can be inhibited, this technique is not intended to solve the above problems related to the high voltage charge type cells.
Patent document 3 discloses such a technique that lithium manganese nickel compound oxide and lithium phosphate are used as the positive electrode. According to this technique, a cell that excels in charge/discharge efficiency at high voltage can be obtained. Patent document 4 discloses such a technique that by adding lithium phosphate to a nonaqueous electrolyte solution that uses LiPF6, the occurrence of hydrofluoric acid is inhibited. However, use of these techniques cannot sufficiently inhibit the elution of cobalt and the decomposition of the electrolyte solution when the positive electrode is charged to a potential higher than 4.3 V based on lithium.